Conventionally, there is known a technique for assisting a driver by monitor-displaying a bird's-eye view image viewed downwards from an upper virtual viewpoint of a vehicle in order to check a blind spot of a self vehicle. This bird's-eye view image is obtained by photographing a vehicle periphery with multiple in-vehicle cameras and effecting viewpoint conversion on these photographed images acquired by the multiple in-vehicle cameras for rendering them into images viewed downwards from an upper virtual viewpoint upwardly of the vehicle and joining these images together. In the course of the above, an ingenious arrangement is provided for allowing the driver to readily recognize the situation of the self vehicle periphery by synthesizing a self vehicle image representing the self vehicle at the center of the bird's-eye view image which is being displayed on the monitor.
A vehicle periphery displaying device using such bird's-eye view image is described in Patent Document 1. With this vehicle periphery displaying device, a bird's-eye view image of vehicle periphery is displayed based on respective photographed images photographed by multiple cameras. And, this bird's-eye view image is divided into a plurality of areas and a target image corresponding to an area selected by a user is displayed with enlargement. Therefore, since a particular area in the bird's-eye view image selected by the user is displayed with enlargement, there accrues an advantage that only an area of interest in the periphery is displayed with enlargement. However, in the bird's-eye view image from an upper virtual viewpoint obtained through the projective transformation from the photographed images, an object having a certain height from a road surface will appear with significant distortion, which makes it difficult for the driver to recognize the distance and/or the size thereof.
Further, according to a parking assistance device described in Patent Document 2, the device includes an image acquiring means for acquiring an image of periphery of a self vehicle, an image transforming means for generating a bird's-eye view image from an upper virtual viewpoint of the self vehicle from images of periphery of the self vehicle and an image of the self vehicle, and a 3D object detecting means for detecting a 3D object present in the periphery of the self vehicle and calculating a distance of the object from the self vehicle and a relative speed as 3D object information. Further, in case it is found based on the 3D object information that the 3D object is a moving object, the bird's-eye view image is displayed on the monitor with such change of scale reduction that allows this 3D object to be displayed at an end of the bird's-eye view image. More particularly, in displaying of the bird's-eye view image, the bird's-eye view image is displayed in enlargement with change of the scale reduction that causes the 3D object to be displayed at the end of the bird's-eye view image relative to the self vehicle as the center, when the self vehicle and the 3D object in the periphery approach each other. However, even with this device, the device only enlarges a portion of the bird's-eye view image. Therefore, in this enlarged image, the object having a certain height from the road surface will appear with significant distortion, which makes it difficult for the driver to recognize the distance and/or the size thereof.
Also known is an image synthesis device attempting to reduce image distortion by generating a projection image obtained by further effecting a projective transformation using a three-dimensional projection plane consisting of a spherical face and a cylindrical face on the projection image using a two-dimensional projection plane constituting the road surface plane, when photographed images obtained by multiple cameras having differing viewpoints are to be synthesized with each other. However, with this device, the displaying image which is finally displayed is a projection image using the three-dimensional projection plane, there is a problem that it is not possible to secure sufficiently straightness of a recognition symbol on road surface such as a white line which was possible with the bird's-eye view image from the upper virtual viewpoint.